This invention relates to a waveguide containing layered particles having lateral dimension of less than 1 micrometer arranged in a concentration gradient and to a method of making the same, and a display screen employing same.
Optical screens typically use cathode ray tubes (CRTs) for projecting images onto the screen. The standard screen has a width to height ratio of 4:3 with 525 vertical lines of resolution. An electron beam is scanned both horizontally and vertically across the screen to form a number of pixels which collectively form the image.
Conventional cathode ray tubes have a practical limit in size, and are relatively deep to accommodate the required electron gun. Larger screens are available which typically include various forms of image projection. However, such screens have various viewing shortcomings including limited viewing angle, resolution, brightness, and contrast, and such screens are typically relatively cumbersome in weight and shape. Furthermore, it is desirable for screens of any size to appear black in order to improve viewing contrast. However, it is impossible for direct view CRTs to actually be black because they utilize phosphors to form images, and those phosphors are non-black.
Optical panels used for viewing images may be made by stacking waveguides. Such a panel may be thin in its depth compared to its height and width, and the cladding of the waveguides may be made black to increase the black surface area. It is known in the art that waveguide componentis utilized for transmission of light. It is further known in the art that a waveguide has a central transparent core that is clad with a second material of a lower refractive index. In order to provide total internal reflection of light within this waveguide, the central core has a higher refractive index of refraction than the clad. By adjusting the difference in refractive index the acceptance angle of incoming light may be varied. The larger the difference in refractive index, the larger the incoming light acceptance angle.
In related work, U.S. Pat. No. 6,307,995 discloses a gradient refractive index in a planar optical waveguide in which the core material contains fluorinated polymer, silicone, silica, polytetrafluoroethylene and other materials. While this patent discloses certain concentration gradients, there is no practical disclosure as how to make a gradient.
However, optical waveguides of the step index cladding type have some significant drawbacks. In the formation of a large optical panel using stepped index clad waveguides many layers are stacked on top of each other and adhered to each other. In a typical 50 xe2x80x3 diagonal screen there may be several hundreds or even thousands of waveguides that are adhered to one another. Handling and cutting many strips of thin polymer is very difficult. The compatibility of materials that have a refractive index difference from core to clad is limited. This may contribute to problems such as inadequate adhesion between layers. Such incompatibility may result in layer to layer interface problems such as air gaps or rough surface or layer separation. These types of problem may cause a loss of light at each bounce at the interface between the core layer and surrounding cladding layers. Although the loss of light at each bounce within the optical waveguide may be small, a light ray may undergo a large number of bounces as it traverses the core layer. Therefore, the amount of light loss that occurs in optical panels becomes a significant detriment to the overall efficiency and performance of the optical panel, as well as the quality such as brightness, and sharpness of the image. When there is a discrete step or boundary between the core and cladding of a waveguide, it is important to control the angle of the incoming light. Light entering a waveguide at acute angles typically will penetrate deeper into the cladding layer than those entering at more oblique angles and therefore it has a higher probability of being scattered resulting in light loss. It would be useful if there was a way to have light within a waveguide turn in a gradual manner and therefore minimize losses due to scattering.
Since there are a limited number of materials that can be used in combination between the core and the clad that provide the desired delta refractive index, adequate adhesion between the layers and can absorb ambient room light, it is important to have a means of controlling or modifying the refractive index of polymers to assure that both optical and physical characteristics are optimized. In stepped refractive index clad waveguides of the type described in U.S. Pat. Nos. 6,002,826, 6,301,417 and 6,307,995 it is important to control or modify the refractive index difference between two different materials or modify the refractive index of the same polymer. If the difference is too large, the ambient light acceptance of the screen becomes large and does not appear to be black. There remains a need for improved control of refractive index as well as a broader selection of materials that can be used.
Another drawback of using optical waveguides of step index cladding type is as follows. When light entering a core layer comprises two or more different wavelengths, a phenomenon known as chromatic dispersion results. Each wavelength portion of light will travel at a slightly different speed and may result in the light exiting the waveguide core at slightly different angles resulting in poor color quality of the image. This means that the exit angle of the light at the outlet face of the optical panel is dependent on the wavelength, or color, of the components of the input light. As can be envisioned, this phenomenon is further exaggerated when the light path that a light ray follows through the core layer increases. The chromatic dispersion that occurs in optical panels using optical waveguides of step index cladding type is another significant detriment to the performance of the optical panel, as well as the quality (e.g. color, sharpness, etc.) of the image.
Ever since the seminal work conducted at Toyota Central Research Laboratories, polymer-clay nanocomposites have generated a lot of interest across industry. The utility of inorganic nanoparticles as additives to enhance polymer performance has been well established. Over the last decade or so, there has been an increased interest in academic and industrial sectors towards the use of inorganic nanoparticles as property enhancing additives. The unique physical properties of these nanocomposites have been explored by such varied industrial sectors as the automotive industry, the packaging industry, and plastics manufacturers. These properties include improved mechanical properties, such as elastic modulus and tensile strength, thermal properties such as coefficient of linear thermal expansion and heat distortion temperature, barrier properties, such as oxygen and water vapor transmission rate, flammability resistance, ablation performance, solvent uptake, etc. Some of the related prior art is illustrated in U.S. Pat. Nos. 4,739,007; 4,810,734; 4,894,411; 5,102,948; 5,164,440; 5,164,460; 5,248,720; 5,854,326; and 6,034,163.
In general, the physical property enhancements for these nanocomposites are achieved with less than 20 vol. % addition, and usually less than 10 vol. % addition of the inorganic phase, which is typically clay or organically modified clay. Although these enhancements appear to be a general phenomenon related to the nanoscale dispersion of the inorganic phase, the degree of property enhancement is not universal for all polymers. It has been postulated that the property enhancement is very much dependent on the morphology and degree of dispersion of the inorganic phase in the polymeric matrix.
The clays in the polymer-clay nanocomposites are ideally thought to have three structures: (1) clay tactoids wherein the clay particles are in face-to-face aggregation with no organics inserted within the clay lattice; (2) intercalated clay wherein the clay lattice has been expanded to a thermodynamically defined equilibrium spacing due to the insertion of individual polymer chains, yet maintaining a long range order in the lattice; and (3) exfoliated clay wherein singular clay platelets are randomly suspended in the polymer, resulting from extensive penetration of the polymer into the clay lattice and its subsequent delamination. The greatest property enhancements of the polymer-clay nanocomposites are expected with the latter two structures mentioned herein above. Most of the work with nanoclays has been for physical properties modification. Therefore, the need exists for a waveguide that can be made with finite control of the refractive index between the core and the clad that will provide a broader selection of materials that can be used.
There is a continuing need to improve waveguides that have efficiency problems due to light loss as well as problems with chromatic dispersion. The present invention solves problems experienced in the prior art, such as the decrease in efficiency, performance and quality resulting from the light loss from the discreet bounces that the light undergoes in the optical waveguides of step index cladding type, and the adverse affects of chromatic dispersion when using optical waveguides of step index cladding type, by providing a plurality of planar optical waveguides for an optical panel, the planar optical waveguides comprising a core material having a gradient refractive index.
The invention provides an optical component comprising an elongated channel for light travel comprising a light transmitting polymeric central core and further comprising multilayer particles, wherein a majority of the particles have both a longest dimension less than 1 micrometer and an aspect ratio of longest to smallest dimension of from 1000:1, to 10:1, wherein the particles are arranged in a concentration differential in at least a portion of a plane normal to the length of the channel so as to create a refractive index gradient in that plane. The invention also provides a method of imaging using the waveguide and a display incorporating the waveguide.
The invention provides a means to convey light with improved efficiency.